1. Field of the Invention
This invention relates to a method of measuring a junction temperature of a semiconductor device, and more particularly to a method of measuring a junction temperature from the current/voltage characteristic of a Schottky diode.
2. Description of the Art
The junction temperature is defined as the temperature at various junctions (e.g., PN junction, etc.) within a semiconductor device, and is calculated by the sum of the environmental temperature and a temperature elevation which is due to the Joule heat of the junction. The junction temperature is one of the rated values of a semiconductor device. When the junction temperature exceeds a rated value, an increase in a leakage current, lowering of the long-term reliability, or breakdown may take place. For these reasons, measuring the junction temperature is an indispensable procedure in designing ICs, or constructing a system by using ICs.
As a method of measuring such junction temperatures of the semiconductor device is known as a diode method. For example, in the measurement of the junction temperature, which is necessary for conducting failure or fault analysis of a LED driver ICs, Schottky diodes for monitoring the temperature are provided at several portions within the IC pattern. This is because the forward voltage of the diode is very sensitive to changes in the junction temperature, and regularly varies with good reproducibility of results. By making use of this feature, some diodes are also used a temperature sensors.
The following formula is a formula for determining the current/voltage characteristics in a forward direction of the Schottky diode. ##EQU1##
In formula (1), I.sub.F is a forward current, S is an area of Schottky contact, A is an effective Richardson constant, T is an absolute temperature, q is the charge of electron, .PHI..sub.B is a Schottky barrier height, k is a Boltzmann constant, V.sub.F is a forward voltage, and n is an ideal factor.
Transformation of the formula (1) produces: ##EQU2## In formula (2), k, q and A are constants, and .PHI..sub.B, S and n are constants that are peculiar to individual diodes. Although .PHI..sub.B and n have a temperature dependency, these parameters can be considered to be substantially constant in a range from room temperature to about 150.degree. C. Namely, since all of the parameters, except for the absolute temperature T, can be considered to be substantially constant in formula (2), it is possible to determine the absolute temperature (junction temperature) T from the forward voltage V.sub.F.
Generally, in semiconductor devices including compounds such as GaAs, etc., Schottky barrier diodes that utilize a potential barrier which is produced at the contact portion of the metal electrode and the semiconductor are widely used. However, since the characteristic of the Schottky barrier diode is dependent upon the state of the surface (i.e., interfacial state) between the metal electrode and the semiconductor, and since it cannot be stated that uniformity or homogeneity of the compound semiconductor substrate is better than that of a silicon semiconductor substrate, there are great variations in the characteristics thereof.
The state of surface (i.e., interfacial state) is reflected in the ideal factor n in formula (2). In formula (2), n is related to all of the terms, and the differences in n are reflected in the forward voltage V.sub.F. In order to facilitate the understanding of this concept, differentiating formula (2) by the absolute temperature T produces: ##EQU3## The change of the ideal factor n in formula (3) is reflected by the temperature coefficient T.C. of the forward voltage V.sub.F. Namely, if characteristics of the surface varies for any reason, so that the values of the ideal factors n are different for various diodes, the temperature coefficients T.C. of the forward voltages V.sub.F would also vary. Such a phenomenon is not rare, instead, it is quite ordinary for this to take place.
FIG. 1 is a graph showing measured temperature dependency curves of the forward voltages V.sub.F in connection with three Schottky diodes that were taken from the same wafer. The temperature dependency curves of the forward voltages V.sub.F of diodes 1 to 3 are different. This is based, in part, on the above-described differences in the state of the surfaces (interfacial state).
Since the forward voltage V.sub.F as stated above, does not have a predetermined relationship to the temperature thereof, it was necessary to: (1) change the ambient temperature; (2) measure dependency in the forward voltage V.sub.F with respect to temperature; and (3) thereafter perform the measurement of the junction temperature T in view of the dependency.
Since the measurement of the temperature dependency of the forward voltage V.sub.F takes too much time, this was an obstruction to implementation of efficient measurement of the junction temperature.